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Tuesday, November 7, 2017
Coralgal reef morphology records punctuated sea-level rise during the last deglaciation
Interesting work. What it shows us is another target for gathering geological time data across deep time well into the Pleistocene and prospectively for millions of years as well if enough data were collected. We rely on the reality that ocean depth is global and the liklihood signatures can be established as well.
This is ahuge empirical task but we are now up to it.
The carbon aspect may surprise us as well. At least the data itself is plausibly secure and repeatable and easily avoid contamination.
Coralgal reef morphology records punctuated sea-level rise during the last deglaciation
reefs preserve the signatures of sea-level fluctuations over Earth’s
history, in particular since the Last Glacial Maximum 20,000 years ago,
and are used in this study to indicate that punctuated sea-level rise
events are more common than previously observed during the last
deglaciation. Recognizing the nature of past sea-level rises (i.e.,
gradual or stepwise) during deglaciation is critical for informing
models that predict future vertical behavior of global oceans. Here we
present high-resolution bathymetric and seismic sonar data sets of 10
morphologically similar drowned reefs that grew during the last
deglaciation and spread 120 km apart along the south Texas shelf edge.
Herein, six commonly observed terrace levels are interpreted to be
generated by several punctuated sea-level rise events forcing the reefs
to shrink and backstep through time. These systematic and common
terraces are interpreted to record punctuated sea-level rise events over
timescales of decades to centuries during the last deglaciation,
previously recognized only during the late Holocene.
reef establishment and evolution during the last deglaciation have been
well documented through chronological, sedimentological, and
paleontological studies, and provide unique data sets upon which past
sea-level records have been reconstructed1,2,3,4,5,6,7,8,9,10,11,12,13,14,15.
Most of these records display, since the Last Glacial Maximum (LGM),
several major intervals of rapid sea-level rise over timescales of
several centuries, referred to as melt water pulses, in the uppermost
Since the early 1930s16, several deep banks, with crests lying in about 60 mbsl, were known to occur along the south Texas shelf edge (Fig. 1). The coralgal origin of the banks was first proposed in the mid-1970s17,18
based on five banks from which rock samples were collected by piston
coring, dredging, box coring, and Van Veen grab. The rocks consist
mainly of dead corals (Agaricia sp., Madracis sp., Madracis asperula, Madracis brueggemanni, Madracis myriaster, and Paracyathus pulchellus)
and coralline algal nodules. Only two samples were dated; a coral
sample from the top of Dream Bank at 68 m yielded a radiocarbon age of
10,580 ± 155 years BP (11,901.5 ± 335.5 calendar years BP), and a
coralline algal sample from the base of Southern Bank produced a
radiocarbon age of 18, 900 ± 370 years BP (22,361 ± 428 calendar years
In late 1990s, a multi-channel seismic grid on one of the reefs,
Southern Bank, indicates the thickness of the bank to be about 40–50 m19.
It is also concluded that the drowned banks along the south Texas shelf
edge were established on paleo highs associated with antecedent
siliciclastic topographies such as either beach barrier islands or beach
ridges developed during late LGM or earliest deglaciation19.
In absence of detailed chronologic dates and based upon the current
water depth range of these bank tops at about 60 mbsl, the demise of
these reefs was proposed to have occurred between ~12,250–11,500 Cal BP.
Recent studies show that during the LGM, the south Texas coastal system
consisted of a bay bounded by the Rio Grande and Colorado lowstand
shelf edge deltas, isolated from the open ocean by a barrier island
complex20 (Fig. 1b).
The coralgal reefs likely established themselves on top of this
lowstand coastal system, thrived, and grew vertically in less than ~8000
years by tracking the 40–50 m of sea-level rise during the uppermost
Ultimately, the south Texas reefs drowned and, starting at ~9 ka, were
subsequently partially buried by the Holocene Texas Mud Blanket17,18,19,20,21 (TMB).
observed 40–50 m vertical accretion of the coralgal banks in about 8000
years suggests average rates of sea-level rise of 5–6 m per millennium,
as in published sea-level records1,2,12,13. This pace could have occurred only with the occurrence of scleractinian coral species, including Acropora palmata and Acropora cervicornis, which display unusually fast growth rates and create the main coral framework of the Caribbean reefs22.
Although these species are not currently growing at the latitudes of
the northern Gulf of Mexico (GoM), except for a few colonies of A. palmata newly established at the Flower Garden Banks (FGB) (Fig. 1a) in the past decade23,
it is assumed that these species formed the coral framework of the
south Texas shelf edge drowned banks. This assumption is bolstered by
the recent discovery that A. palmata and A. cervicornis grew in large numbers at the base of the FGB24
as early as 10,200 cal BP, based on radiocarbon dating. The occurrence
of these coral species as early as the earliest part of the Holocene in
the northern GoM strengthens the inference that they most likely form
the coral framework of the uppermost Pleistocene south Texas shelf
drowned banks. Additionally, modern and presumably deglacial
near-surface circulation patterns in the GoM show that it is and was
responsible for carrying biotic communities into the GoM from the
has been established that carbonate production areas shrink through
backstepping so to remain within the euphotic zone when responding to
sea-level rise; as such coralgal reefs form distinct sets of terraces26
as they grow vertically keeping up with sea-level rise. During
transgressions, therefore, episodic and rapid sea-level rise events
result in set of terraces, preserving the nature of sea-level rise and
diagnostic morphological features of reefs struggling to keep up with
Ultimately, when the area of carbonate production has shrunk to a
minimum through systematic backstepping, reefs are unable to grow
vertically fast enough to keep up so as to remain within the euphotic
zone and reefs ultimately drown26,31.
The edifices of drowned reefs sit below the euphotic zone, as the
series of drown banks along the south Texas shelf edge, which are no
more vertically accreting, although their crests are still covered by
live ahermatypic wire corals, sea-fans, mollusks, annelids, bryozoans,
and red algae, and are known to be excellent fishing grounds32.
new data presented here provides an opportunity to quantify well-imaged
back-stepping terraces and identify nature of the sea-level rise during
last deglaciation leading to the development of common backstepping
morphologies. High-resolution multibeam mapping and seismic profiling of
10 drowned banks, located along a 120-km-long stretch of the south
Texas outer shelf, identify six common terrace levels; these identical
morphologies provide new opportunities to understand coralgal reef
evolution through backstepping and terrace formation, most likely
triggered by decade to century-long punctuated sea-level rise during the
middle part of last deglaciation. Existing sea-level records do not
have the ability to resolve these smaller amplitude variations. Hence,
it is pertinent to investigate geological records that directly document
spatiotemporal sea-level changes to determine if decadal to
century-scale sea-level rise episodes are common occurrences.
True coralgal reef morphologies
Multibeam bathymetric mapping and 3.5 kHz seismic profiling, acquired in September 2012, onboard the R/V Falkor (Fig. 2 and Supplementary Figs. 1–6),
showcase the detailed morphological architecture of the south Texas
shelf edge drowned banks. Spurs and grooves, typical morphological
adaptations to high-energy inner fore reef conditions33,34 (Figs. 2a and 3a, b),
are preferentially observed in the high-resolution bathymetry on the
south-eastern margins of several mapped banks and, therefore, coincident
with their windward high-energy sides; on their protected north-western
lee sides, these features are conspicuously absent. In mid-1970, spurs
and groves were already observed, aligned perpendicular to the slope of
the bank, by submarine operations using DRV Diaphus35. Moreover, the new data presented here provides an opportunity to quantify well-imaged backstepping terraces (Fig. 3c–e),
defined as flat areas bounded by steep slopes, common in nine of the
ten surveyed banks. These terraces, separated by 1–2 m high faces of
coralgal reef rock as was previously observed using submersibles35,
are quantitatively analyzed based on multibeam data. Additionally,
Dream Bank displays narrow-rimmed margins enclosing shallow lagoons at
two different backstepping terrace levels, typical coralgal atoll
morphologies (Fig. 3c).
Ultimate coralgal reef demise
57.5–61.8 mbsl depth range in which the crests of eight of the ten
drowned coralgal reefs occur, point to their contemporaneous demise
(Fig. 3f). Furthermore, this depth range coincides with stranded paleo-shorelines and subtidal shoal complexes observed in the GoM36 (~58 mbsl), Caribbean37 (~57 mbsl), and Southwest Pacific37
(~56 mbsl). These paleo-shorelines and shoals are interpreted to have
been abandoned by an ~11.5 ka event of rapid rise in global sea level,
linked to the onset of MWP-1B occurring at the end of the Younger Dryas1,15.
It is hypothesized, therefore, that the final demise of the south Texas
drowned banks was triggered by the MWP-1B, at ~11.5 ka. The coralgal
reefs could not keep up26,31
with the rapid rise in sea level because their carbonate production
surface areas had shrunk to a minimum through systematic backstepping,
as an overall response to the last deglaciation sea-level transgression.
other than sea-level rise can negatively affect coralgal community
growth, such as fluctuations in water turbidity, temperature, and
salinity. However, siliciclastic sediment influx into the south Texas
shelf edge was minimal during the uppermost Pleistocene transgression20,
when coastlines migrated landward. Initial burial by the TMB was
initiated at ~9 ka, and thereby post-date reef drowning by ~2.5 ka.
Temperature and salinity likely did not trigger the widespread collapse
of the south Texas banks. During the Younger Dryas, sea surface
temperatures dropped only by ~1.5 °C to reach 26 °C, and sea surface
salinity increased from 34 to 36.5 parts per thousand in the northern
These nominal changes likely did not modify the coralgal reef ecology
because during the time period of reef development, sea surface
temperatures and salinity are estimated to have fluctuated with an even
greater magnitude, between 25 and 29 °C, and 34–38 parts per thousand,
Terrace hypsometric analysis
curves, generated from eight banks, identify sets of backstepping
terraces at uniform water depths, within a range spanning 75–60 mbsl.
Four individual terraces are identified at: 74 ± 1, 70.5 ± 1.5,
66.5 ± 1.5, and 63 ± 1 mbsl. The terraces are separated by 2–4-m-high
steep face. As imaged in 3.5 kHz seismic lines, a fifth well-developed
common terrace, buried by the TMB, is identified at 82 ± 1 mbsl (Figs. 4 and 5a and Supplementary Figs. 1–6).
Moreover, a sixth terrace was mapped at 94 ± 1.5 mbsl only on Harte
Bank—(the deepest bank with an exposed crest and base at 82 and
102 mbsl, respectively; Supplementary Fig. 7), discovered during the 2012 research expedition aboard the R/V Falkor
cruise. Because both subsidence and glacio-isostatic adjustment (GIA)
rates are assumed to be identical along this 120 km of the south Texas
shelf, the observed five common terrace depth ranges can be considered
coeval. Despite the absence of systematic chronologic dates for each of
the terraces, their consistent depth ranges, among several reefs growing
over such this long stretch of the south Texas shelf edge, are
interpreted to reflect contemporaneous and systematic backstepping
linked to punctuated sea-level rises.
Paleo terrace depth estimates
changes are dependent on ice-sheet growth and decay, tectonics, and
sediment overloading of the shelf and vary in different parts of the
world, referred to as relative sea-level (RSL). RSL curves incorporate
eustatic sea-level (ESL) fluctuations and it is usually difficult to
separate the two (RSL and ESL). The Northwestern GoM is an ideal
location for which RSL drivers and their amplitudes are well constrained
and provides the opportunity to examine ESL signals. The two main
drivers for RSL change in northern GoM are GIA39 (0.71 mm per year of uplift since 21,000 calendar years BP), and subsidence40
(0.5 mm per year from past 21,000 years). Considering a linear rate for
GIA and subsidence for the last deglaciation, and the current depth of
the terraces on drowned banks, the depth of each terrace is recalculated
and used as indicator of ESL (Table 1). The corrected depths are compared with an ice-volume sea-level curve15
to calculate the corrected age range with uncertainties, during which
each of the six terraces was developed. These calculations are based on
two assumptions: the GIA and subsidence rates are linear, and the
development of terraces occurred at sea level. The uncertainties
associated with the age model are dependent upon the GIA, subsidence,
and relation of paleo water depth to terrace depth. Calculating GIA and
subsidence with corrected age model demonstrates that the deviation in
the GIA and subsidence are less than three percent. Further, atoll and
spur-groove morphologies, clearly observed in the high-resolution
bathymetric data sets, indicate that the reefs, when flourishing, were
keeping up with sea level.
Paleo terrace depths and Greenland climate record
corrected depths of the observed six common terrace levels, identified
on the south Texas banks, are projected onto a global eustatic sea-level
curve15 (Fig. 5b)
and their equivalent ages with uncertainties are estimated based on
these projections. Then, these ages with their associated uncertainties
are projected onto the NGRIP δ18O record41,42.
This climate record from Greenland is, to our knowledge, the only
existing high-resolution upper Pleistocene climate record during which
the six terraces were formed along the south Texas shelf. The comparison
of both records is the only possible opportunity to attempt to
understand the cause and effect relationship between warm climate
intervals, melting of glaciers (ice-stream/ice-sheet collapse),
sea-level rise events, and terrace development. As observed in Fig. 5b,
out of the six terraces, four terraces correspond to warm
interstadials, one to a stadial–interstadial transition, and one only to
a cold stadial. The NGRIP δ18O record represents climate variations in Greenland. Figure 5b
further illustrates that the number of occurrence of the terrace depth
zones are similar to the number of warm events observed on NGRIP δ18O
record. Moreover, the correlation of each terrace to a warm
interstadial period, with one exception, is noteworthy. These warm
periods, therefore, can further be linked to ice-sheet/ice-stream
collapse events, causing rapid sea-level rise events of the orders of
few meters per century, which lead to the development of common terrace
morphologies on south Texas shelf banks.
absence of correct chronologic dates, the formation of these terraces,
common to nine coralgal reefs, located along a distance of over 120 km
on the south Texas shelf edge, indicates that during the recent peak
deglaciation sea level did not always rise gradually, but rather was
characterized by a series of punctuated and rapid sea-level rise events
over decades to one century, previously only recognized during late
Holocene43,44. Because climate warming and resulting ice-sheet collapses have been predicted for the future decades and centuries45,46,
the steady and gradual sea-level rise, observed over the past two
centuries may, therefore, not be a complete characterization of how sea
level would rise in the future. Furthermore, there is a scientific need
to utilize advanced technologies, including high-resolution bathymetry
systems combined with systematic drilling of reefs and accurate dating
techniques; this study serves as a guide to future research endeavors
that seek to inform sea-level rise rate and amplitude projections.
Researchers that model sensitivity of sea-level fluctuations—past and
present—require as much information as possible regarding smaller
amplitude events, and the best place to find this information is from
the geological record. The documentation of decades to century-scale
punctuated sea-level rise events with magnitudes of a few meters implies
that deglaciation and associated sea-level rise is a non-steady
process. Rate of sea-level rise has been observed to accelerate since
the past two decades47;
therefore, these results have significant implications for the
community of science researchers that examine sea-level rise past and
present, and for how society prepares for coastal flooding and
inundation hazards in the coming decades to centuries.
Radiocarbon date calibration
Calib Rev 7.0.4 was used to calibrate the radiocarbon ages collected in 1970s17,19,21. Calibration data set marina13.14c is used with Delta R = −30 ± 9.
The new calibrated calendar year ages are 11,901.5 ± 335.5 calendar
years BP for the top of Dream Bank and 22,361 ± 428 calendar years BP
for the base of Southern Bank. These ages are not incorporated into the
age model but are used to only indicate that these reefs grew during
Data collection R/V Falkor
During a 15-day long research cruise in September 2012 onboard R/V Falkor,
funded by the Schmidt Ocean Institute, high-resolution multibeam sonar
and 3.5 kHz seismic data sets were acquired over 10 drowned coralgal
reefs on the south Texas shelf edge. The research vessel was equipped
with state-of-the-art instrumentation, including a Kongsberg EM 710
multibeam echo sounder to collect high-resolution (<0 .5="" 10.1="" 320="" 3260="" 7.1="" a="" analyzed="" ancillary="" and="" arc="" attitude="" bathymetric="" build="" caris="" chirp="" cnav="" components="" correction="" data="" drowned="" echo="" further="" g.i.s.="" heading="" high-resolution="" image="" imported="" included:="" investigate="" khz="" knudsen="" m="" mapping="" maps="" multibeam="" of="" p="" positioning="" post="" processed="" profiler.="" reefs.="" sea-floor="" seabed="" seapath="" sedimentary="" seismic="" sensor="" service="" software.="" sub="" survey="" sv="" system="" the="" these="" to="" units.="" using="" utilizing="" valeport="" was="">0>
Hypsometric curves—data analysis
Hypsometric curves are generated for nine of the ten drowned coralgal reefs (Fig. 5a).
Bathymetry data sets are clipped into subdata sets encompassing each
individual drowned reef. The total surface area of each reef is divided
into 1 m-depth intervals and the surface area of each interval is
calculated by the number of pixels (each one representing one square
meter). To create a hypsometric curve for each individual reef, the
percentage of each one meter depth interval is determined using their
calculated surface area. Each peak in a given reef’s hypsometric curve
represents individual terrace. When the nine hypsometric curves are
plotted together, overlapping peaks identify common terrace depth zones.
For each common terrace depth zone, a median terrace depth is
determined. Depth uncertainties are evaluated as the difference between
the median terrace depth values and their maximum or minimum depth
Computing paleo terrace depth
In the absence of chronologic dates, the current depths of the terrace zones, ice-volume sea-level curve15, in addition to GIA39 and subsidence40
rates, are used to estimate paleo terrace depths. First, the terrace
depth zones (with depth uncertainties) are compared with an ice-volume
to identify the age range (including uncertainties) for the development
of each terrace zone. To estimate the paleo terrace depth, depth change
for each terrace due to GIA and subsidence is calculated by multiplying
the estimated age range of each terrace depth zone with avg. rate of
GIA39 (0.71 mm per year) and subsidence40
(0.5 mm per year). The total change in depth is calculated by adding
GIA and subsidence (uplift is considered positive and subsidence is
considered negative). The estimated total depth change is added to the
current terrace depth to identify the paleo terrace depth for each
terrace. The paleo terrace depths are further compared with an
ice-volume sea-level curve15 to estimate the age range for the development of each paleo terrace.
Two assumptions are included in the analyses: the rates of subsidence40 (0.5 mm per year) and GIA39 (0.71 mm per year) are considered linear and 95% probability ice-volume sea-level curve15 is used.
Digital data generated by the environmental sensor systems onboard R/V Falkor,
including multibeam, are archived and freely available to the public
via Rolling Deck to Repository and given Digital Object Identifiers—http://www.rvdata.us/catalog/FK005B
All relevant data are available from authors.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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